1. Field of the Invention
The present invention relates to engine control for internal combustion engines and, more particularly, for small internal combustion engines of the type which are used in a variety of applications, such as walk-behind lawnmowers, lawn and garden implements, generators, or in small utility vehicles such as riding lawnmowers, lawn tractors, and the like.
2. Description of the Related Art
Small internal combustion engines generally include an operator-selected command speed setting, for example, a throttle control for utility vehicles or a normal/idle switch for generators. However, driving a variable load may reduce or increase the engine speed from the commanded setting. For example, in a lawnmower powered by an internal combustion engine, it is desired that the commanded speed of the engine remain relatively constant under a variety of loading conditions. Thus, it is desired that whether the lawnmower encounters tall grass or short grass, the engine speed which has been selected by the operator should remain constant. Likewise, in the case of a generator, it is desired that the alternator output frequency, i.e., the engine drive speed, remain constant despite changes in the electrical loads connected to the alternator output.
To regulate engine speed, small internal combustion engines generally include a mechanical speed-regulating governor, such as an air vane mechanism or a centrifugal flyweight mechanism sensitive to engine speed. For engines having a carburetor, the throttle valve is generally mechanically linked to both the governor and the operator throttle control. Therefore, the throttle valve is acted upon by a first force related to the commanded speed setting and a second force corresponding to the governor and related to the actual engine speed.
A disadvantage of known engine control systems for small internal combustion engines is the potential unreliability of cables, springs, and linkages that are used to transmit and combine the inputs from the operator-commanded engine speed and the actual engine speed. Such components might bind, require lubrication, or may fail from mechanical vibrations or loading.
Another disadvantage of known engine control systems for small internal combustion engines is the difficulty of mechanically adjusting the amount of movement of the throttle valve as it relates to the commanded engine speed setting or the actual engine speed and the difficulty of providing dampening of transients due to engine speed changes.
Yet another disadvantage of known speed control mechanisms for small internal combustion engines is that ambient temperature and engine operating conditions are not taken into account to adjust the fuel-to-air ratio for easy starting and optimum efficiency for a range of ambient engine conditions.
What is needed is an engine control system for internal combustion engines that reliably accounts for the commanded engine speed setting and the actual engine speed to drive the throttle and fuel controls, and that accounts for the ambient and engine operating temperatures to provide an efficient fuel-to-air ratio.